rare inductor needed where find ? 0.7uH

[nickthevoice]

HI guys

I search anywhere on google - ebay - digikey - solen.ca and I don't find anywhere this amplifier output INDUCTOR value it's 0.7uH or 700nH for output of LM780 board.

OR Can I change this RL circuit (10ohm resistor / 0.7uH) by RC ?

thanks in advance !

nicK

 

[hpeter]

https://www.elfa.se/elfa3~tr_en/elfa/init.do?item=58-080-14&toc=20335
7A enough?

 

[AndrewT]

Get a AA battery. Or any mandrel of similar diameter.
Get a length of 1mm diameter enamelled copper wire. 200mmm gives just enough for short tails.
Wind 10Turns around the battery.
Slide the copper turns off the battery.
You now have a ~0.7uH air core inductor

 

[nickthevoice]

thanks !!!

thanks hpeter but I don't know if the sound is good with this type of inductor VS air core ?

thanks ANDREWt it's a good idea but I don't have inductor tester

I try this anyway !


THANKS !

nicK

 

[DF96]

It is very unlikely that the inductor value has to be exact. There are inductor design tools on the internet, and various formulae. If you carefully make an inductor you can expect that it will be within about 10-20% of the intended value.

 

[Mark Johnson]

10 Turns AWG-22 insulated, solid core wire wound around a 1.5V AA battery. Measured on Thurs 28 March, 1700 GMT. (1300 EDT in New York City).

Measured inductance was 696 nanoHenries on an AADE LC meter

 

[GoatGuy]

Nice! However it would have been over 700 nH had the coils been more tightly adjacent to each other. [DF96]'s point stands: its hardly likely that the exact value is critical. GoatGuy

 

[nickthevoice]

WOW THANKS TO ALL FOR YOUR QUICK REPLY !

I have crossover inductor in stock and I take that wire it's 20 AWG

but do you glue or other tips to get all wire loops very close together ?
on the pics of GOATGUY the loop is not close but it work well too

nicK

 

[GoatGuy]

but do you glue or other tips to get all wire loops very close together ?
on the pics of GOATGUY the loop is not close but it work well too

Just start with REAL STRAIGHT wire. If its wiggly, bent, not really straight, you're not going to get nice close windings. But if its straight-straight, then by hand you can wind this and have it look beautiful, in a minute or two.

If you're using magnet wire, stripping is a problem that has a not-so-obviously solution. Alcohol flame. Burns off the enamel, and you can quench the hot copper in another diluted bit of alcohol. Work on an area that is going to be safe for flaming alcohol (its not overly dangerous, just ... things happen). You can even make a quick-and-dirty alcohol burner from a piece of aluminum-foil about 3cm x 3cm. Wrap it over the tip of your finger (or that AA battery!) and make a small cup. fill with 5mm to 10mm isopropyl alcohol, and put in 1 little wad of glass wool (from wall/ceiling insulation).

It'll burn for awhile, long enough to do the stripping. Its pretty safe, and you can just chuck it when you're done. the problem with commercial isopropyl alcohol is that it is 30% water. So, the lamp becomes "water logged", and won't burn after a few minutes. Safe!

GoatGuy

 

[Mark Johnson]

RF guys use "Coil Dope" to keep their hand-wound coils mechanically stable, so the inductance value and the Q remain constant. Google shows a number of homebrew recipes for making Coil Dope at home; the top two appear to be

• Polystyrene ("styrofoam") packing peanuts dissolved in toluene or acetone
• Clear glossy polyurethane varnish (for wood floors or wood boats)

Here is a cool website where they tried 4 different ways of stabilizing a coil, plus a fifth coil as a control. The before-and-after experimental results are:

 

[bear]

Ummm... REMOVE THE BATTERY!!

It's just for winding!!

If you want a form to keep the coil of wire on, use plastic or ceramic... glass is ok too...
wood is a last resort and should be varnished first, imo.

_-_-bear

 

[lcsaszar]

That inductor is not critical at all. It is put on the output of the amplifier for provide stability in case of capacitive load of the speaker cable. You can find it at the output of many transistor amplifier with negative feedback. It is usually realized by winding 5-6 turns of insulated wire on a 5W resistor. It is not an off-shelf item. Trust me, it has no audible effect...

 

[mondogenerator]

i know this is an old thread...

Ive just tried this.

No 1mm gauge to hand, so i used 1.5mm, 17 turns, wrapped around a pencil. Using handheld LCR meter they both read 0.63 uH (nulling out lead inductance). Varnished using thinned epoxy resin, two turns of glasstape, then varnished again.

I'll measure DCR on a 10 Amp resistance bridge tomorrow, and post the value.

Just for info purposes, and in case it helps out anyone, who wants to do the same.

 

[AndrewT]

if you want lower resistance then use a MUCH larger diameter former.
an AA battery is bigger. A C battery is bigger still.
8T to 10T on an AA battery is about 0.7uH.